Optical information processing apparatus including a correction circuit for correcting non-linearity of an output of a position detection circuit

ABSTRACT

An optical information processing apparatus includes a unit for scanning a track formed on an optical recording medium with a light beam, a tracking control unit for correcting a position shift between a radiation position of the light beam on the medium and the track, and an offset correction unit for automatically correcting an offset of the tracking control unit.

This application is a divisional of application, Ser. No. 07/562,021,filed Aug. 2, 1990, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical information processingapparatus for recording and/or reproducing information on/from anoptical recording medium such as an optical disk.

2. Related Background Art

Recently, a "digital signal processing method" of performing digitalsignal processing in place of conventional analog processing has becomepopular, and is widely used in practical applications such as compactdisks (CDs), digital audio tapes (DATs), communication lines, and thelike. The "digital signal processing method" has many merits since acomplicated signal processing algorithm can be realized by software. Forexample, a hardware system can be simplified, system cost can bereduced, changes in specifications such as selection of filterconstants, algorithms, and the like can be flexibly made, and so on. Inaddition, along with the progress in IC techniques, inexpensive digitalsignal processing integrated circuits (ICs), and digital signalprocessors (DSPs) are commercially available.

An optical information processing apparatus such as an optical disk hasrapidly progressed as a high-density, large-capacity memory from acompact disk (CD) player to a rewritable magnetooptical disk apparatusvia a direct-read-after-write (DRAW) type apparatus. In particular, amagnetooptical disk apparatus is required to have high reliability and ahigh access speed in addition to the above-mentioned high-density,large-capacity features as an external memory of a computer.

For this reason, an electrical servo system requires control as acomplicated combination of outputs from various sensors. Amagnetooptical disk apparatus will be exemplified below with referenceto FIG. 1.

In FIG. 1, a light beam emitted from a semiconductor laser 1 iscollimated by a collimator lens 2, and the collimated beam is convertedby a polarization beam splitter 3 with a beam shaping function into alight beam having a substantially circular section. The collimated beamis reflected by a prism 4, and is then incident on an object lens 5. Theobject lens 5 is movable in a focus direction 6 and a tracking direction7 by an actuator (not shown), and focuses a small light spot on a disk9. A magnetooptical recording layer is formed on the disk 9. A directionof an arrow 10 corresponds to a direction of tracks, and an arrow 11indicates the center of rotation of the disk. The prism 4, the objectlens 5, the actuator, and the like are fixed on a carriage 13, and thecarriage 13 can be moved in a radial direction of the disk 9 using alinear motor (not shown), and the like.

Light reflected by the disk 9 is converted to a collimated beam by theobject lens 5, and is deflected by the prism 4 toward the polarizationbeam splitter 3. The light beam is then reflected by the polarizationbeam splitter 3 toward a direction of a detection optical system, and issplit by a beam splitter 16 through a focusing lens 15 into a light beamreflected toward a servo sensor 18 and a light beam transmitting towardradio frequency (RF) sensors 19 and 20.

The focusing lens 15 includes an element for generating, e.g., anastigmatism, and the light beam is focused on the servo sensor 18. Theservo sensor 18 comprises four-split sensors 18-1 to 18-4. The servosensor 18 is aligned in directions of three axes while observing that alight spot is focused on a predetermined track on the disk 9. The sensor18 is then adjusted to obtain equal outputs from the four sensors.

The light beam transmitting through the beam splitter 16 is split intotwo beams by a polarization beam splitter 17, and the two beams arerespectively focused on the RF sensors 19 and 20. The semiconductorlaser 1, the collimator lens 2, the RF sensors, and the like are fixedto a head fixing unit 14. The magnetooptical disk apparatus shown inFIG. 1 employs a so-called divided optical system which is divided intothe carriage 13 and the head fixing unit 14, and can allow high-speedaccess.

An actuator unit of the magnetooptical disk apparatus will be describedbelow with reference to FIG. 2.

In FIG. 2, the object lens 5 is fixed to a bobbin 21. A tracking coil 22and a focusing coil 23 drive the bobbin 21 in the tracking and focusdirections 7 and 6 in cooperation with a tracking magnet 24 and afocusing magnet 25 fixed to a yoke 26. The bobbin 21 is supported by asupport shaft 27. An under limiter 28 determines the lowermost end ofthe bobbin. A counterweight 29 of the object lens 5 is fixed to thebobbin.

A light-emitting diode 30 is fixed to a flexible printed board 31. Alight beam emitted from the light-emitting diode 30 is shaped via a slit32, and the shaped light beam is projected onto a 2-split sensor 34 as alight beam 33. The light-emitting diode 30 is fixed to the bobbin 21.When the actuator is shifted in the tracking direction, amounts of thelight beam 33 incident on light-receiving surfaces 34-1 and 34-2 of the2-split sensor are changed, and the outputs from these surfaces can becalculated to detect a position of the object lens 5. The 2-split sensor34 is connected to a flexible printed board 35.

A linear motor unit of the magnetooptical disk apparatus will bedescribed below with reference to FIG. 3.

In FIG. 3, an actuator including the object lens 5, the bobbin 21, themagnet 24, the yoke 26, and the bobbin support shaft 27 is fixed on thecarriage 13. The carriage 13 is supported on rails 36-1 and 36-2through, e.g., bearings 37-1 and 37-2, and is movable in a disk radialdirection 12. The linear motor unit comprises a coil 38, a yoke 39,magnets 40-1 and 40-2, and the like. In this case, linear motors areattached to two sides of the carriage to allow high-speed access. Aspindle motor 41 rotates the disk.

A servo system for the magnetooptical disk apparatus described abovewith reference to FIGS. 1 to 3 will be described below with reference toFIG. 4.

The servo sensor 18 is adjusted to obtain equal outputs from the foursensors 18-1 to 18-4 when the object lens 5 is located at the center ofa light beam from the semiconductor laser 1 and the light beam forms asmall spot of about 1 micron on a track of the disk 9. In this case,since a focus error detection method employs an astigmatism method, ifthe outputs from the sensors 18-1 to 18-4 are represented by S₁ to S₄, adifference between outputs of diagonal sums is observed, thus obtaininga focus error signal S_(AF) given by:

    S.sub.AF =(S.sub.1 +S.sub.3)-(S.sub.2 +S.sub.4)

For example, when a light spot is in an in-focus state on the disk, theabove-mentioned output becomes 0. When the light spot is in a near-focusstate on the disk, a negative output is obtained; when the light spot isin a far-focus state, a positive output is obtained.

A tracking error detection method employs a push-pull method. In thepush-pull method, a balance of diffraction light from a guide groove ofa disk is observed in a far field. A distribution of diffraction lightis unbalanced according to a radial position shift between apredetermined track on a disk and a light spot. Thus, a differencebetween outputs of the sensors divided by a dividing line along atangential direction of the sensor 18 is observed, and a tracking errorsignal S_(AT) given by the following equation is obtained:

    S.sub.AT =(S.sub.2 +S.sub.3)-(S.sub.1 +S.sub.4)

For example, when the light spot is located on a track, the output iszero. When the light spot is shifted in an inner peripheral direction ofthe disk, a negative output is obtained; when the light spot is shiftedin an outer peripheral direction of the disk, a positive output isobtained.

In the push-pull method, when the object lens 5 is largely shifted in aradial direction (tracking direction) by, e.g., a multi-track jump mode,since the light beam focused on the servo sensor 18 is moved in theradial direction, an auto-tracking (to be abbreviated as AT hereinafter)output is offset in addition to an unbalanced distribution ofdiffraction light according to the above-mentioned track shift. In orderto perform high-speed access, it is advantageous that the object lenscan be used to be moved by about 100 to 150 tracks from the center of alight beam from the semiconductor laser 1. Since this offset almostcorresponds to a shift amount of the object lens from the center of alight beam, it can be easily corrected as long as the object lensposition can be detected.

In this case, the object lens position detection means (to be referredto as a lens sensor hereinafter) as described in FIG. 2 is arranged. Theoutputs from the two sensors 34-1 and 34-2 are respectively representedby S_(LP1) and S_(LP2), and these outputs are adjusted so that a lensposition (to be abbreviated as LP hereinafter) output S_(LP) given bythe following equation becomes 0 when the object lens is located at thecenter of the light beam:

    S.sub.LP =S.sub.LP1 -S.sub.LP2

When the object lens 5 is located at the center of the light beam, theabove-mentioned output becomes 0. However, when the object lens isshifted in an inner peripheral direction of the disk, a positive outputis obtained; when it is shifted in an outer peripheral direction of thedisk, a negative output is obtained.

Since the LP sensor output represents a position shift between thecarriage 13 and the object lens 5, a linear motor can be driven usingthis data, so that the object lens position can always be kept at thecenter of the light beam.

The servo sensor and the like have been described. However, it isimpossible to perfectly mechanically align these components. Whenconventional analog servo signal processing is performed, a sensoroutput is normally electrically adjusted by a control volume (not shown)after mechanical adjustment.

Servo signal processing will be briefly described below.

The outputs S₁ to S₄ from the servo sensor 18 are amplified by apreamplifier 43, and then are output from an arithmetic unit 44 as ATand auto-focus (to be abbreviated as AF hereinafter) outputs, asdescribed above. The outputs S_(LP1) and S_(LP2) from the LP sensor 34are amplified by a preamplifier 45, and then are output from anarithmetic unit 46 as the LP output. Of these outputs, the AT and LPoutputs are added to each other by an adder 47 to be corrected, so thatthe tracking error signal is not offset even if the object lens positionis shifted (corrected AT output). The AF output, the corrected AToutput, and the LP output are supplied to a digital signal processingcircuit 48, and are then output respectively to AF, AT, and linear motordrivers 49, 50, and 51 at proper timings. These drivers output drivesignals to the AF, AT, and linear motor coils 23, 22, and 38,respectively, thus executing focus control and tracking control.

An RF system will be described below with reference to FIG. 5.

The RF system shown in FIG. 5 includes the RF sensors 19 and 20described above. Preamplifiers 52 and 53 respectively amplify outputsfrom the RF sensors 19 and 20. Amplifiers 54 and 55 calculate adifference and a sum of the outputs from the RF sensors 19 and 20. Amagnetooptical signal output 56 is detected as a difference between theoutputs from the RF sensors 19 and 20 in such a manner that rotation ofa plane of polarization of a light beam caused by a magnetoopticaleffect is detected by the polarization beam splitter 17. A preformatsignal 57 representing, e.g., a sector mark or an address corresponds toa linear increase/decrease in light amount incident on the RF sensors 19and 20, and is detected as a sum of the outputs from the RF sensors 19and 20.

A magnetooptical disk will be described below with reference to FIG. 6.

In FIG. 6, tracks 58 and guide grooves 59 are concentrically or spirallyarranged to have a disk center 11. Each track is divided into headerareas in each of which preformat signals such as sector marks andaddresses are recorded in advance in the form of pits 60, and data areasin each of which magnetooptical signals are recorded by a user in theform of magnetooptical pits 61.

In the magnetooptical disk apparatus with the above-mentionedarrangement, since sensor outputs are electrically adjusted by, e.g., anadjusting volume, this results in a cumbersome operation, and it isdifficult to reduce manufacturing costs of the apparatus.

Since the above focus or tracking error detection method does notdirectly detect a focusing state of a light spot on a disk, when arelative position between the servo sensor and a light spot on the servosensor or a position of the semiconductor laser is shifted by anyexternal force after adjustment, the light spot can no longer becorrectly focused on a predetermined track. These changes in states mayoften occur due to a change in temperature or a vibration duringtransportation, and servo precision is impaired. Even when the positionsensor itself does not suffer from a position shift, the wavelength ofthe semiconductor laser may be changed due to a change in temperature.As a result, a deflection angle of a light beam in the polarization beamsplitter with the beam shaping function is changed, and a light spotposition on the servo sensor is undesirably moved.

An AT offset which occurs when the center of the object lens is shiftedfrom an optical axis in a so-called multi-track jump mode wherein theobject lens is moved in the tracking direction to move a light beam toanother track separated from the present track by several tracks ischanged due to a variation in depth of the guide groove. Therefore, thisresults in degradation of tracking precision unless an offset value isadjusted for every disk.

Japanese Laid-Open Patent Application No. 53-129604 discloses an opticalinformation processing apparatus which can automatically correct an AFoffset.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the aboveproblems, and has as its object to provide an optical informationprocessing apparatus which is free from complicated adjustmentprocesses, and can automatically correct a position shift of opticalparts, and the like. The present invention is suitable for digitalcontrol, and can simultaneously attain cost reduction and improvement ofservo precision, which cannot be attained by conventional analogcontrol.

In order to achieve the above object, according to the presentinvention, there is provided an optical information processing apparatuscomprising:

means for scanning a track formed on an optical recording medium with alight beam;

tracking control means for correcting a position shift between aradiation position of the light beam on the medium and the track; and

means for automatically correcting an offset of the tracking controlmeans.

According to the present invention, a focus control means for correctinga disk surface displacement is calibrated so that an optimal offset forfocusing a light spot on an optical disk is added to a focus errorsignal, and its focus gain is also calibrated to obtain optimal servostability. A first tracking control means for correcting a small diskeccentricity at a relatively high frequency is calibrated so that anoptimal offset for precisely tracking a light spot on a predeterminedtrack position is added to a tracking error signal, and its trackinggain is calibrated to obtain optimal servo stability. A means for, whenan object lens for focusing a light spot is shifted from an optical axisin a tracking direction, detecting the position of the object lens iscalibrated by counting the number of tracks of a given disk. In order tocorrect a tracking error occurring when the object lens is shifted fromthe optical axis in the tracking direction, the relationship between theobject lens position and the offset value of the tracking error signalis calibrated by counting the number of tracks of a given disk. A gainof a second tracking control means for correcting a large diskeccentricity at a low frequency or moving an object lens position in aradial direction of a disk is calibrated to obtain optimal servostability using a given disk and the object lens detection means.

An optical information processing apparatus employing an automaticcontrol device for a servo system according to the present inventioncomprises an optical disk loading detection means, and executes theabove-mentioned calibration operations every time a new disk is loaded.The optical information processing apparatus using an automatic servoadjustment method of the present invention comprises a temperaturedetection means. When a change in temperature exceeding a predeterminedvalue occurs and servo precision is in doubt, new calibration operationsare executed using the given optical disk as described above. Theabove-mentioned operations can be easily attained by using a digitalprocessing circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view for explaining an optical systemof a magnetooptical disk apparatus;

FIG. 2 is an exploded perspective view for explaining an actuator of themagnetooptical disk apparatus;

FIG. 3 is a schematic perspective view for explaining a linear motor ofthe magnetooptical disk apparatus;

FIG. 4 is a block diagram for explaining a servo system of themagnetooptical disk apparatus;

FIG. 5 is a block diagram for explaining an RF system of themagnetooptical disk apparatus;

FIG. 6 is a schematic plan view for explaining an arrangement of themagnetooptical disk apparatus;

FIGS. 7(A, B) is a block diagram showing an embodiment of a controlcircuit used in an optical information processing apparatus according tothe present invention;

FIG. 8 is a flow chart showing an automatic control sequence in theapparatus according to the present invention;

FIG. 9 is a chart for explaining a correction method of an offset valueof a tracking error signal used in the present invention;

FIG. 10 is a chart showing an output of an object lens position sensorused in the present invention;

FIG. 11 is a chart for explaining a method of detecting an eccentricityof a disk using the lens position sensor;

FIG. 12 is a chart for explaining the first embodiment of a method ofcalibrating a lens position sensor according to the present invention;

FIG. 13 is a chart for explaining the second embodiment of a method ofcalibrating a lens position sensor according to the present invention;

FIG. 14 is a chart for explaining a method of calibrating an offsetvalue of a tracking error signal when an object lens of the presentinvention is shifted from a reference position;

FIG. 15 and FIGS. 16A and 16B are charts for explaining the firstembodiment of offset correction of a focus error signal according to thepresent invention;

FIGS. 17A and 17B are charts for explaining the second embodiment ofoffset correction of a focus error signal according to the presentinvention;

FIG. 18 is a chart for explaining the first embodiment of a method ofadjusting an AF gain according to the present invention;

FIGS. 19(A, B) is a chart for explaining the second embodiment of amethod of adjusting an AF gain according to the present invention;

FIG. 20 is a chart for explaining linearity correction of a laser powermonitor according to the present invention; and

FIG. 21 is a flow chart for explaining an algorithm for embodying thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 7 is a block diagram showing an embodiment of a control circuitused in an optical information processing apparatus according to thepresent invention.

The apparatus of the present invention is substantially the same as thatdescribed with reference to FIGS. 1 to 6 except for this circuit.

A servo sensor 18, a preamplifier 43, and an arithmetic unit 44 areconnected as shown in FIG. 7. The arithmetic unit 44 is connected to adigital signal processing circuit 48 via an input switching circuit 62and an A/D (analog-to-digital) converter 63. The arithmetic unit 44 isalso connected to a track counter 64. The output of the track counter 64is connected to the digital signal processing circuit 48. A lensposition sensor 34 is connected to the input switching circuit 62 via apreamplifier 45. The output of a home position sensor 65 is connected tothe digital signal processing circuit 48. A CPU (central processingunit) 66 is bidirectionally connected to the digital signal processingcircuit 48 and an external interface 67. A spindle motor 42 for rotatinga disk is connected to the digital signal processing circuit 48 via amotor driver 68. Two RF sensors 19 and 20 are connected to an RF signalprocessing circuit 69 via preamplifiers 52 and 53. One output of the RFsignal processing circuit 69 is connected to the input switching circuit62 via a detector 70, and the other input thereof is connected to thedigital signal processing circuit 48. A memory 71 for storing variousdata is connected to the digital signal processing circuit 48. Thedigital signal processing circuit 48 is connected to a laser diode 1, afocus coil (AF coil) 23, a tracking coil (AT coil) 22, and a linearmotor coil 38 through a D/A (digital-to-analog) converter 72 and anoutput switching circuit 73, four sample & hold (S/H) circuits 74, 75,76, and 77, and then drivers 78, 49, 50, and 51. A monitor photodiode 79for monitoring light emitted from the laser diode is connected to theinput switching circuit 62 via a preamplifier 80. A temperature sensor81 for detecting temperature in the apparatus is connected to the inputswitching circuit 62.

The basic operation of the circuit shown in FIG. 7 will be describedbelow.

A light beam incident on the servo sensor 18 is converted into a voltagesignal by the preamplifier 43. Thereafter, a focus error signal, atracking error signal, and a focus/tracking sum signal are calculated bythe arithmetic unit 44 based on the voltage signal. One of these signalsis selected by the input switching circuit 62, and the selected signalis converted into a digital signal by the A/D converter 63. The digitalsignal is input to the digital signal processing circuit 48. The digitalsignal processing circuit 48 outputs a digital control value to the D/Aconverter 72 to control the AT and AF coils so that tracking and focuserror levels become zero. An analog control signal output from the D/Aconverter 72 is selected by the output switching circuit 73, and is thenheld by the S/H circuits 75 and 76. Thereafter, the held signals areoutput to the drivers 49 and 50. The drivers 49 and 50 respectivelydrive the AF and AT coils 23 and 22.

In order to read/write a magnetooptical signal, a laser beam must beradiated on a disk. The digital signal processing circuit 48 outputs alaser beam control value to the D/A converter 72. The analog-convertedsignal is selected by the output switching circuit 73, and is then inputto the laser driver 78 via the S/H circuit 74. The laser driver controlsthe laser diode 1 so that a light amount necessary for read/write accesscan be obtained. The monitor photodiode 79 for monitoring light emittedfrom the laser diode is attached to the laser diode, and the output fromthe photodiode 79 is input to the input switching circuit 62 via thepreamplifier 80. Since the light amount is monitored by the monitorphotodiode 79, the digital signal processing circuit 48 can accuratelycontrol a laser output light amount. A signal line directly connectedfrom the digital signal processing circuit 48 to the laser driver 78 isa high-speed laser ON/OFF signal line used in a write mode.

The lens position (LP) sensor 34 comprises a 2-split photodiode, and isirradiated with light emitted from an LP sensor light-emitting diode(LED) 30. An output from the photodiode is changed upon a change inobject lens position. This output is amplified by the preamplifier 45,and the amplified signal is input to the input switching circuit 62. Thesignal is then input to the digital signal processing circuit 48 via theA/D converter 63. An output from a home position sensor 65 for detectingthat an actuator is moved to a home position on an outer periphery sideis input to the digital signal processing circuit 48.

The CPU 66 for managing the overall sequence operation according to thepresent invention is connected to the digital signal processing circuit48 to control the operation of the circuit 48. The CPU 66 is alsoconnected to the external interface 67 to manage data exchange with anexternal device.

The memory 71 stores various data supplied from the digital signalprocessing circuit 48 or the CPU 66 via the circuit 48.

Rotation of the spindle motor 42 is controlled by the motor driver 68.The start and stop operations of the spindle motor 42 are controlled bythe CPU 66 via the digital signal processing circuit 48.

The linear motor coil 38 is driven through the driver 51 in response toa speed command from the digital signal processing circuit 48. When thelinear motor is started, the tracking error signal from the arithmeticunit 44 appears as a track crossing signal. The number of trackingsignals during movement of the linear motor is counted by the trackcounter 64 to detect a moving track count. The digital signal processingcircuit 48 calculates a target moving speed, and the like on the basisof a target track count and a present track count.

The two RF sensors 19 and 20 convert a magnetooptical signal and apreformat signal into electrical signals. These signals are amplified bythe preamplifiers 52 and 53, and the amplified signals are thensubjected to difference detection, in-phase detection, and peakdetection processing operations in the RF signal processing circuit 69.The output from the RF signal processing circuit 69 is processed, asdigital data, by the CPU 66 via the digital signal processing circuit48, and the processed signal is output, as digital information, to theexternal device via the external interface 67. On the other hand, theenvelope of the signal subjected to the RF signal processing is detectedby the detector 70 as an analog signal, and the signal is then input asa signal indicating a magnitude level to the digital signal processingcircuit 48 via the input switching circuit 62 and the A/D converter 63.This signal is used to judge the magnitude of the RF signal level and todetect whether or not focus and tracking operations are normallyexecuted.

FIG. 8 shows an automatic control sequence of the servo system in theapparatus of the present invention.

First, the object lens is positioned at the center of a light beam fromthe semiconductor laser, and only the AF servo is operated. Then, anoffset value of a tracking error signal is measured and corrected (step1). The offsets to be corrected at this time include an alignment errorupon adjustment of, e.g., the servo sensor, a position shift afteradjustment, warp of a disk, and the like.

Next, the object lens position sensor is calibrated, and a trackingerror signal offset when the object lens is shifted from the center ofthe light beam is corrected (step 2). These two operations can beperformed at the same time but may be performed independently. Theobject lens position sensor output is calibrated based on an absoluteobject lens position from the center of the light beam which is detectedby counting the number of tracks of the disk. Thus, linearity of theobject lens position sensor is corrected.

Correction of the tracking error signal offset executed when the objectlens is shifted from the center of the light beam is performed tocorrect linearity between the object lens position and the offset valueof the tracking error signal, which occurs due to the causes describedin step 1. At the same time, a variation in offset caused by a variationin, e.g., depth of the guide groove of the disk is corrected.

A focus error signal offset is then corrected (step 3). This step may beexecuted before step 2. The AF and AT servo operations are performed andan offset value is determined to maximize reproduction amplitudes ofpreformat signals (e.g., sector marks, address signals, and the like) ofthe disk. Thus, an AF offset caused by an alignment error uponadjustment of, e.g., the servo sensor, variations in thickness andrefractive index of a disk substrate, a variation in the guide groove ofthe disk, and the like can be corrected.

Then, an AF gain is adjusted (step 4). The AF and AT servo operationsare performed to add a proper focus disturbance from the digitalprocessing circuit, and a response thereto is measured, therebyadjusting the gain to a predetermined value. Initial and agingvariations of the actuator, variations in the disk, and the like can becorrected at the same time.

An AT gain is adjusted in the same manner as in step 4 (step 5).

A linear motor gain is adjusted (step 6). The AF and AT servo operationsare performed on a predetermined track to add a proper disturbance fromthe digital signal processing circuit to the linear motor, and aresponse from the linear motor is measured using the object lensposition sensor calibrated in step 2. Initial and aging variations ofthe linear motor can be corrected.

Finally, laser power monitor linearity is corrected based on the monitorphotodiode incorporated in the semiconductor laser (step 7). Since themagnetooptical disk apparatus uses a laser power by changing it to havea difference of about 10 times between a data reproduction mode and dataerase and write modes, the monitor linearity is impaired by lightreflected by the disk. Thus, the poor linearity can be corrected by theoutput from the servo sensor. Thus, a recording/reproduction operationwith an optimal laser power can be performed.

The correction methods in respective steps will be described in detailbelow.

METHOD OF CORRECTING TRACKING ERROR SIGNAL OFFSET AT OBJECT LENSREFERENCE POSITION (ON OPTICAL AXIS)

In order to prevent already recorded data from being erased duringcorrection, the carriage is moved to its home position. The homeposition sensor 65 comprising, e.g., a photointerrupter, and amechanical switch shown in FIG. 7 can detect whether or not the carriageis moved to the home position. The object lens is then moved to thecentral position of a light beam from the semiconductor laser (to bereferred to as a lens reference position hereinafter). As a method ofattaining this operation, when a focus actuator is moved downward to alowermost point, a mechanical pin may be engaged at a central point.Alternatively, the output from the preamplifier 45 of the LP sensor 34is adjusted in advance to have a predetermined value at the lensreference position in the manufacture, and the lens position may beelectrically moved so that the LP sensor output has the predeterminedvalue.

In this state, a focusing operation is performed to set the lens at asubstantially focal point. Then, a tracking error signal upon trackcrossing is generated by the following methods. In one method, thelinear motor coil 38 is energized while the lens is fixed in position atthe reference position to vibrate the linear motor. When the linearmotor is sinusoidally vibrated, the object lens is vibrated to cross thetracks, and a tracking error signal ((S₂ +S₃)-(S₁ +S₄)) can be obtainedfrom the arithmetic unit 44. In the second method, the object lens isslightly vibrated in the tracking direction at the reference positionwhile the linear motor stands still at the home position. In thismanner, the tracking error signal including an offset can be obtainednear the lens reference position.

As shown in FIG. 9, the tracking error signal has an offset component.This signal is a tracking signal output from the arithmetic unit 44, andis A/D-converted based on a sampling pulse shown in FIG. 9 via the inputswitching circuit 62. Thus, the digital signal is input to the digitalsignal processing circuit 48. The digital signal processing circuit 48obtains peak and bottom values from the digital tracking signal, andthen obtains an intermediate point of these values, thereby recognizingthis point as an offset value. In order to more accurately obtain thepeak and bottom values, the tracking error signal is preferably sampledseveral times. The offset value obtained in this manner is stored in thememory 71. In a tracking operation after correction, the obtainedtracking offset value is subtracted from the digital tracking errorsignal before offset correction, which is obtained from the arithmeticunit 44 via the A/D converter 63 to generate an offset-correctedtracking error signal after offset correction. Then, tracking loop iscontrolled using the offset-corrected tracking error value.

CALIBRATE OBJECT LENS POSITION SENSOR

The outputs of the lens position sensor 34 have characteristics so thatthe two sensor outputs S_(LP1) and S_(LP2) change in opposite directionswith respect to an object lens position shift, as shown in FIG. 10.Basically, the following arithmetic operation is performed to remove anin-phase fluctuation such as a temperature fluctuation of the sensoroutput, thus detecting the object lens position:

    (S.sub.LP1 -S.sub.LP2)/(S.sub.LP1 +S.sub.LP2)

However, since the outputs S_(LP1) and S_(LP2) are not linearly changedwith respect to the object lens position, the relationship between thesensor outputs and the object lens position must be detected by thefollowing methods.

First Method

The object lens position is set at the center of a light beam from thesemiconductor laser, focus servo and tracking servo are set in in-focusand on-track states, and a disk is then rotated. Since the disk suffersfrom an eccentricity, the tracking actuator fluctuates in the trackingdirection to follow the eccentricity, and the two outputs S_(LP1) andS_(LP2) of the lens position sensor 34 vary accordingly. As shown inFIG. 11, the fluctuating outputs S_(LP1) and S_(LP2) are sampled inresponse to a rotation synchronous sampling pulse which is synchronizedwith rotation of the disk, and the sampled outputs are converted intodigital signals by the A/D converter 63. Thus, eccentricity data duringone revolution are stored in the memory 71 via the digital signalprocessing circuit 48. These data are used to remove an eccentricitycomponent during sampling of the object lens position data (to bedescribed below).

A tracking servo loop is then opened to jump the tracking actuator by anobject lens moving range (e.g., ±250 microns=±170 tracks). During thisoperation, data of the relationship between the outputs S_(LP1) andS_(LP2) of the lens position sensor and the object lens positiondisplacement are sampled. While the object lens position is moved from a-170th track position to a +170th track position, the outputs LP1 andLP2 are sampled at every eleventh track, and are A/D-converted. Theoutputs S_(LP1) and S_(LP2) fluctuate under the influence of theeccentricity, as shown in FIG. 12. Thus, data obtained by subtractingthe eccentricity component from the sampled data by utilizing theabove-mentioned eccentricity data to remove the eccentricity are storedin the memory 71.

Second Method

In the first method, the object lens position is continuously moved fromthe -170th track position to the +170th track position, and data aresampled during movement. In this method, however, the object lensposition is jumped by several tens of tracks, and the tracking loop isclosed to sample data. The same operation as in the first method isexecuted until the object lens position is brought to the opticalcentral position to enable the tracking loop. In the second method,however, no eccentricity data are sampled. In this method, the objectlens position outputs S_(LP1) and S_(LP2) are loaded during one or aplurality of revolutions of the disk, and average values of the outputsS_(LP1) and S_(LP2) are obtained by the digital signal processingcircuit 48, thus obtaining the object lens position output from which aneccentricity component is removed. As shown in FIG. 13, a track jumpoperation is executed by the predetermined number of tracks, and thetracking loop is closed at the object lens position after movement.During one or several revolutions of the disk, the object lens positionoutputs are loaded, and their average values are obtained, therebyobtaining the object lens position output at that point. In this manner,the track jump operation, data sampling, and average value calculationsare repeated, and object lens position output values free from aneccentricity component over the entire object lens moving range arestored in the memory 71.

Third Method

In the first or second method, data sampling is performed while theobject lens position is continuously moved or the track jump operationis performed. In this method, data sampling is performed by tracing. Theobject lens position is jumped inwardly by 170 tracks and the trackingloop is closed, Since the spiral grooves are formed in the disk from itsinner periphery toward the outer periphery, the object lens positiontraces from the inner periphery toward the outer periphery in thisstate. The object lens position outputs are sampled in every revolutionduring tracing. In this manner, since data sampling is performed inevery revolution, no eccentricity component is caught, and data samplingfree from an eccentricity component can be automatically performed.

When data sampling of the relationship between the object lens positionand the LP sensor output is completed by any one of the first to thirdmethods and the sampled data are actually used, the object lens positionmust be obtained from the object lens position outputs. In one method, aconversion table may be allocated in the memory 71. In this case,however, a numerical arithmetic method using a digital signal processor(DSP) or the like which can perform high-speed arithmetic operationswill be described below.

Basically, a lens position is approximated by a quintic equation.

    Position=A·(X+B·X.sup.2 +C·X.sup.3 +D·X.sup.4 +E·X.sup.5)

where X is the normalized object lens position output, and A, B, C, D,and E are constants. That is, we have: ##EQU1## where G and K areconstants. G is selected so that the range of X corresponds to ±1.0 whenthe values of S_(LP1) and S_(LP2) are substituted. A, B, C, D, and E canbe determined based on the values of S_(LP1) and S_(LP2) by the law ofleast squares so that a position error is minimized. K is used tocorrect a difference between output levels of S_(LP1) and S_(LP2). Whenadjustment is performed in advance to yield S_(LP1) =S_(LP2) when theobject lens is located at the lens reference position, K=1 can be set.

In this embodiment, the LP sensor detects the position of the objectlens in a track crossing direction. However, the present invention isapplicable to a case wherein this sensor detects the position of theobject lens in an optical axis direction.

CORRECT TRACKING ERROR SIGNAL OFFSET WHEN OBJECT LENS IS SHIFTED FROMREFERENCE POSITION

Since the tracking error offset value and the object lens positiondisplacement have a linear relationship to some extent, it is possibleto correct the tracking offset using this relationship. In this case,offset correction is executed in the digital signal processing circuit.

However, a method of more strictly correcting an offset will bedescribed hereinafter. When data of the relationship between the objectlens position and the LP sensor output are sampled, a tracking signalobtained when the object lens is shifted from the reference position inthe radial direction is simultaneously observed, and the relationshipbetween the object lens position and the tracking error offset amount isobtained. As shown in FIG. 14, a signal generated upon crossing oftracks is mixed in the tracking error signal. The peak and bottom valuesof the tracking error signal are read, and their central value iscalculated as the tracking error signal. This value may be stored in thememory 71 as a conversion table or an approximation equation andcorrection of the lens position sensor may be obtained to numericallycalculate the central value using a digital signal processor (DSP) orthe like which can perform high-speed arithmetic operations.

The tracking error signal in this case is in a considerably higherfrequency range than an eccentricity component. Therefore, a samplingpulse must have a frequency high enough to sufficiently catch peak andbottom values of the tracking error signal. For example, when only aneccentricity component is sampled, the sampling frequency can be about500 Hz or about 10 times the frequency (50 Hz) of the eccentricitycomponent. However, in order to read a signal generated upon crossing oftracks, a sampling pulse having a frequency of about 10 kHz which is 10times that (about 1 kHz) of the tracking error signal upon crossing oftracks is required.

CORRECT FOCUS ERROR SIGNAL OFFSET

In the first method of correcting a focus error signal offset, an offsetvalue is determined to maximize a reproduction amplitude of a preformatsignal (e.g., a sector mask or an address signal) on a disk.

The AF and AT servo operations are performed, and an amplitude value ofa signal in a preformat area obtained when an offset is forcibly addedto the focus error signal is monitored. This operation will be describedbelow with reference to FIG. 15. In FIG. 15, an AF offset amount isplotted along the abscissa, and an amplitude value of a signal isplotted along the ordinate. Assume that the amplitude of the preformatsignal obtained when a predetermined positive offset amount is added(point P₃ in FIG. 15) to an initial AF offset position (point P₁ in FIG.15) as the center is a value indicated by x in FIG. 16A, and theamplitude obtained when a negative offset amount is added (point P₂ inFIG. 15) is a value indicated by y in FIG. 16B. The two amplitudes x andy are stored in the memory, and are compared with each other. In thiscase, since x>y, a maximum point of the preformat signal amplitudevalue, i.e., a just focus point is present on the positive side from thepresent position.

In FIG. 15, assume that a point defined by adding a predeterminedpositive offset value to the point P₃ is set to be a new central point.Furthermore, the amplitude value of the preformat signal at a point P₅defined by adding a predetermined positive offset value to the point P₄is stored in the memory, and is compared with the stored value of theamplitude of the preformat signal at the point P₃. Since the amplitudevalue at the point P₅ is larger than that at the point P₃, it isdetermined that the just focus point is present on the further positiveside. In this manner, this operation is repeated to search that the justfocus point is present between the points P₄ and P₆.

The predetermined offset amount is set to be 1/2 that of the initialvalue to narrow a search range. The same operation is repeated to havethe intermediate point P₅ between the points P₄ and P₆ to converge anoffset amount to the just focus point. This operation is continued untila difference from the preformat signal amplitude to be compared becomeszero. The obtained focus offset amount is stored, and is kept applied tothe focus error signal. Note that a differentiated signal of thepreformat signal using a differential circuit (not shown) is preferablyused to improve a detection sensitivity of the just focus point.

As another method of detecting an amplitude value of the preformatsignal, the following methods are known.

(a) In this method, photocurrents from the RF sensors 19 and 20 areamplified by the preamplifiers 52 and 53, outputs from these amplifiersare directly monitored, and a peak value at that time is held to detecta DC component.

(b) In this method, the outputs from the RF sensor preamplifiers 52 and53 are differentiated by a differential circuit (not shown) to detect apeak value of a signal. A p--p value of the differentiated signal ismonitored to detect the amplitude value.

(c) In this method, the differentiated signal output is half-waverectified or full-wave rectified and this peak value is monitored todetect the amplitude value.

(d) In this method, a filter for extracting a certain range where afluctuation of the AF offset amount considerably appears in afluctuation of the amplitude value is used, and the output from thisfilter is monitored.

These amplitude value data are A/D-converted and accessed, and are thenprocessed in the digital signal processing circuit 48.

In the second method, magnetooptical signal data in a data area of thedisk is directly accessed, and its amplitude value can be monitored. Thesequence of this method is the same as that of the first method.

A signal for changing an offset amount, as shown in FIG. 17B, is addedto the focus error signal, and the differentiated magnetooptical signaloutput from a differential circuit, as shown in FIG. 17A, may bemonitored. In this case, a voltage value of an AF offset applicationsignal at a position where the amplitude value of the magnetoopticalsignal is maximized is read (corresponding to a point P in FIG. 17B),and this value is always applied to the focus error signal, thus settinga just focus state.

AUTO FOCUS GAIN CONTROL

The first method of auto focus gain control will be described below withreference to FIG. 18. FIG. 18 is a pseudo block diagram of a processingsequence in the digital signal processing circuit 48. The AF and ATservo operations are performed to set the object lens at the referenceposition, and one track is followed or a track tracking state is set. InFIG. 18, the focus error value (an offset has been removed in theabove-mentioned process), and a sum signal value are digital data afterA/D conversion, and an output value and an estimated value are alldigital data. A disturbance value which has the same frequency as a 0-dBcrossing frequency of the auto focus loop gain and does not cause anerror is given. The amplitude of the disturbance value is given by anincrease/decrease in data in the digital signal processing circuit, andits period is also given by (1/crossing frequency). Amplitude data at anode B after application of the disturbance value is compared withamplitude data before application at a node A by a divider 90. When B<Aor B>A, a value K in a multiplier 91 is adjusted to yield A=B, therebyadjusting a gain.

The second method can be executed even when the digital signalprocessing circuit 48 is limited to a gate array in FIG. 19. Anamplitude value B after application of a disturbance from an oscillator82 is compared with an amplitude value A output from the gate array, andgain control is performed to yield A=B. In this case, C after the outputswitching circuit 73 may be used in place of B. Read values of A and Bhave different phases and cannot be read at the same timing. Therefore,one period of a disturbance is sampled to detect amplitude values A andB, and a comparator 84 for comparing these values, and a gain settingcircuit 85 for causing the gate array to control the gain are separatelyarranged.

Gain control may also be performed by applying a disturbance to a signaloutput from the A/D converter 63 at the input side of the gate array,and comparing the applied amplitude value and an amplitude value afterthe input switching circuit 62.

AUTO TRACKING GAIN CONTROL

Auto tracking gain control is performed in the same manner as in autofocus gain control.

LINEAR MOTOR GAIN CONTROL

Linear motor gain control is performed as follows. As shown in FIG. 19,a disturbance having the same frequency as a 0-dB crossing frequency inthe linear motor loop gain is applied to the linear motor control 38,and a displacement of the linear motor is detected on the basis of theoutput from the LP sensor.

First Method

A servo operation is performed so that the linear motor is fixed at thehome position. The focus and tracking servo operations are thenperformed so that the object lens is located at the reference position.The digital signal processing circuit 48 generates a digital disturbancesignal, and applies the disturbance to the linear motor coil via the D/Aconverter 72, and the like. The linear motor is vibrated by thedisturbance. However, since the tracking servo operation is performed,the object lens is vibrated in the radial direction of the disk incorrespondence with the movement of the linear motor to maintaintracking. Therefore, the LP sensor also generates an output synchronouswith the vibration. Since the linear motor open loop gain is constantexcept for a mechanical sensitivity of the linear motor, an arithmeticgain can be set so that the displacement of the linear motor has apredetermined value (0 dB at the 0-dB crossing frequency) when apredetermined disturbance amplitude is applied. The digital signalprocessing circuit 48 reads the output value from the LP sensor, andsets the linear motor servo loop gain so that the read amplitude valuehas a predetermined value.

Second Method

In this method, a disturbance is generated by the oscillator 82 arrangedoutside the digital signal processing circuit 48, as shown in FIG. 19.As in the first method, focus, tracking, and linear motor servooperations are performed at the home position. The object lens positionis the reference position. In addition, a disturbance frequency is the0-dB crossing frequency. In this method, the output disturbance signalis accessed by an A/D converter 86, its amplitude is detected by anamplitude value detector 92, and the detected value is estimated by thedigital signal processing circuit 48. The displacement of the linearmotor is detected based on the LP sensor output as in the first method.The digital signal processing circuit 48 determines an arithmetic gainso that the displacement of the linear motor has a predetermined value(0 dB at the 0-dB crossing frequency) when a predetermined disturbanceamplitude is applied. Since this method employs an analog oscillator, anoscillation waveform need not be generated by the digital signalprocessing circuit. Therefore, a software load can be reduced, and ahigh frequency can be easily generated.

Third Method

In this method, the object lens is fixed in a reference position, andthe tracking servo loop is opened. A disturbance is applied to thelinear motor to vibrate it, so that the object lens is vibrated in theradial direction of the disk. The number of tracking error signals uponcrossing of tracks is counted to detect a displacement amount of thelinear motor. The focus and linear motor servo operations are performedat the home position, and the disturbance frequency is the 0-dB crossingfrequency as in the first method. In this case, when an eccentricitycomponent is counted, a detected displacement amount suffers from anerror. Therefore, it is necessary to count only an eccentricitycomponent beforehand without application of a disturbance, and tosubtract the eccentricity component from a count value applied with thedisturbance. Although this method often causes an error of a maximum ofabout one track, no problem is posed as long as a large displacementamount is set.

CORRECT LASER POWER MONITOR LINEARITY

According to the present invention, laser power is controlled bydetecting an output signal from the monitor photodiode 79. However, withonly this operation, since a monitor output is influenced by lightreflected by the disk, power of the laser beam radiated on the diskcannot be controlled with perfect precision.

According to the present invention, linearity is corrected using lightreflected by the disk. Light reflected by the disk is received by theservo sensor 18 to be subjected to current-voltage conversion.Thereafter, the output signal is converted to a sum signal (S₁ +S₂ +S₃+S₄) by the arithmetic unit 44. The sum signal is then A/D-converted,and the digital signal is input to the digital signal processing circuit48. On the other hand, the output from the monitor photodiode 79 isinput to the digital signal processing circuit 48 via the preamplifier80 and the A/D converter 63. As shown in FIG. 20, the digital signalprocessing circuit 48 controls the laser driver 78 to emit a laser beamof 10 mW having relatively good monitor linearity. At this time, if thesum signal is 10 V, the laser output can be given by (sum signal/1,000(W)). Data representing the relationship with the monitor output can besampled while decreasing the laser output so that the sum signal outputis decreased by, e.g., every 0.1 V. Thus, the monitor output can becorrected based on the sum signal output. Correction data is stored inthe memory 71, and the monitor output is corrected based on this data tocontrol the laser power, thus allowing precise laser radiation.

FIG. 21 shows an algorithm for embodying a servo system automatic gaincontrol method according to the present invention.

The automatic gain control of the present invention can be performedevery time a magnetooptical disk is loaded and the magnetooptical diskapparatus is started or every time the temperature sensor arranged inthe apparatus exhibits a change in temperature exceeding a predeterminedvalue in use and a position shift of the optical parts described aboveis feared. When the automatic gain control is performed every time a newmagnetooptical disk is loaded, an alignment error upon adjustment of,e.g., the servo sensor or a position shift after adjustment can beeasily corrected. In addition, a variation in AT offset occurring whenthe object lens is shifted in the radial direction due to a variation ina guide groove of the disk, and variations in AF and AT gains can becorrected. At the same time, an AF offset caused by variations inthickness and refractive index of a disk substrate, an AT offset causedby warp of the disk substrate, and the like can also be corrected.

When automatic control is performed every time the temperature sensorexhibits a change in temperature exceeding a predetermined value, aposition shift of optical parts caused by the change in temperature, aposition shift of a light spot on the servo sensor caused by a change inwavelength of the semiconductor laser, and the like can be corrected.For example, in the magnetooptical disk apparatus shown in FIG. 1,assuming that a beam shaping ratio of the beam shaping prism 3 is set tobe 2 and glass is BK7, a deflection angle of a light beam is about 3 secper change in wavelength by 1 nm. If the focal length of the focusinglens 15 is assumed to be 40 mm, a light spot shift on the servo sensoris about 0.6 micron per change in wavelength by 1 nm. Since thewavelength of the semiconductor laser is changed by 0.3 nm per change intemperature by 1° C., a change in temperature of 30° C. causes a lightspot shift of about 6 microns, and this influences tracking servoprecision. However, if automatic gain control is performed every time achange in temperature reaches 5° C., a position shift can be convergedto a value which poses no problem. Thus, the beam shaping prism need notcomprise an expensive achromatic prism as a combination of a pluralityof kinds of glass.

The automatic gain control for a servo system has been described. Thepresent invention is not limited to the focus error detection method,the tracking error detection method, and the object lens positiondetection method described in the above embodiment. Focus and trackingerrors may be detected by independent detectors.

In the above embodiment, light reflected by a medium is detected.However, when a medium is of a light transmission type, the transmissionlight may be detected to calibrate a control means.

What is claimed is:
 1. An optical information processing apparatuscomprising:an optical head for irradiating a light beam onto an opticalrecording medium provided with a plurality of tracks; an objective lens,mounted on said optical head, for condensing the light beam onto therecording medium; an actuator for moving said objective lens in adirection intersecting the tracks; a position detection circuit fordetecting a relative position between said objective lens and saidoptical head and for producing an output; and a correction circuit forcorrecting non-linearity of the output of said position detectioncircuit with respect to an actual position of said objective lens,wherein said correction circuit comprises sampling means for samplingthe output of said position detection circuit at a predeterminedinterval during which said objective lens intersects a predeterminednumber of tracks while said actuator moves said objective lens in thedirection intersecting the tracks, and a memory for storing datarepresenting a relation between the position of said objective lens andthe sampled output of said position detection circuit.
 2. An apparatusaccording to claim 1, wherein said position detection circuit comprisestwo sensors whose outputs are respectively increased and decreased inresponse to a change in position in one direction of said objectivelens, and a circuit for calculating a difference between the outputsfrom said sensors.
 3. An apparatus according to claim 1, wherein saidcorrection circuit comprises a digital signal processor.
 4. An apparatusaccording to claim 1, further comprising a photodetector for detectingone of a light beam reflected by the recording medium and a light beamtransmitted through the recording medium, and a tracking control circuitfor obtaining a tracking error signal indicating a positional deviationbetween the irradiated position of the light beam on the recordingmedium and the track, from an output of said photodetector.
 5. Anapparatus according to claim 4, wherein the recording medium comprises adisk on which the plurality of tracks are concentrically or spirallyformed, and said correction circuit feeds the tracking error signal backto said actuator every time said objective lens intersects thepredetermined number of tracks to scan the disk by one revolution withthe light beam and stores in the memory a mean value of signals outputfrom said position detection circuit during such a scanning period, as asampling output.
 6. An apparatus according to claim 1, wherein therecording medium comprises a disk on which the plurality of tracks areconcentrically or spirally formed, and said sampling means of saidcorrection circuit samples an output of said position detection circuitwhen the light beam is located at an identical position in thecircumferential direction of the disk.